1. Field of the Invention
This invention relates to thermosetting coating compositions which are suitable for application by powder spray methods and fluidized bed coating processes, and methods for the production of such coating compositions. More particularly, this invention relates to solid, modified polyester resins which are cross-linked by polyepoxide resins and to thermosetting coating compositions comprising said modified polyester resins and polyepoxide resin cross-linking agent, which coating compositions are in the form of a stable free-flowing powder, suitable for application by powder spray methods and fluidized bed coating processes.
2. The Prior Art
Generally, thermosetting curable coating compositions are in the form of a solution of a synthetic resin in an organic solvent serving as a carrier. This solution is applied to the base surface, which may be metal, wood, paper or textile, after which the solvent carrier is evaporated and a synthetic resin film remains, which then may be subjected to a heat-hardening treatment.
The use of organic solvents as a carrier for the synthetic resin has the disadvantage that in most cases the solvents are inflammable or even may give rise to explosions. Moreover some organic solvents are physiologically unacceptable as they endanger the health of operators. In addition, much energy is required for the evaporation of the organic solvent and for the conditioning of the air which must carry away the solvent vapors from the oven in which the coated objects are baked.
For these reasons it has been proposed to replace the organic solvents wholly or partially by water. The use of water as a carrier for the synthetic resin, however, dictates the use of certain types of synthetic resins, which might not impart the required technological properties, such as flow, levelling and gloss to the finished coatings.
In view of the disadvantages of the preceding methods, it has also been proposed to apply high-melting thermoplastic, non-curable coating compositions in the form of a homogeneous powder. This powder is mostly applied by the method known as the fluidized bed method. This method is essentially a dipping process wherein the article to be coated is preheated to a temperature above the melting point of the coating composition and then introduced into a fluidized mass of coating composition powder articles.
Powdered thermosetting, curable coating compositions may be applied to the substrate in the same way, requiring, however, two heat treatments, firstly the preheating of the object to be coated and subsequently the curing treatment of the coated object. For this reason the electrostatic powder spray application method has been developed, in which only one heat treatment is required, namely the curing treatment, and this method has the additional advantage that thinner layers (less than 100 microns) may be applied than in the fluidized bed method. Due to the relatively high viscosity of the molten powder, coatings are provided (even in one treatment) which are substantially thicker than can be obtained by the conventional coating methods using solutions of the coating material. Sharp edges or bends are also much better coated by the powder coating processes.
The formulation of fusible, powdered coating compositions for application by powder spray methods and fluidized bed processes introduces requirements for film-forming materials which are entirely different from those met in the formulation of conventional coatings. It will be clear that the film-forming material must be a solid which can readily be converted into a free-flowing powder and is non-caking under normal storage conditions, and that the material must be capable of fusing at an elevated temperature below its degradation or decomposition temperature.
In practice the manufacture of a thermoplastic powder coating has involved much fewer difficulties than that of a thermosetting powder coating. This is due to the fact that the thermoplastic powder coating after application hardens by simple cooling, whereas a thermosetting system has to undergo cross-linking to obtain the desired film properties. For this purpose it is necessary to introduce a curing agent, cross-linking agent or catalyst into the system, which involves special problems.
Up till now the most important thermosetting powder coating systems are entirely based upon epoxy resins, which have as their main disadvantage poor outdoor durability.
Hence there is still a need for powdered free-flowing, fusible, thermosetting coating compositions which are stable during storage, which can be used in electrostatic powder-spray coating processes, and which have a reduced epoxy resin content and therefore can be heat-cured in an acceptable time to a uniform coating having satisfactory chemical and mechanical properties and an acceptable price.